A FIN price, as carried on order and trade events.
The contract serialises prices as "<tag>:<value>". The tag selects the
source variant:
fixed:<decimal>— an order-pool limit priceoracle:<int>— an order-pool oracle deviation, in bpsccl:<decimal>— a concentrated-liquidity (range) effective rate<address>:<rate>— a market-maker offer, prefixed by its contract address